Mechanism of norepinephrine-induced lipolysis in isolated adipocytes: evidence for its lipolytic action inside the cells

Abstract

Norepinephrine elicited remarkable lipolysis in rat fat cells, but did not stimulate lipolysis in a fat cell homogenate. We prepared endogenous lipid droplets from fat cells and established a norepinephrine-sensitive cell-free system consisting of the lipid droplets and hormone-sensitive lipase (HSL). In the cell-free system, hydrolysis of the lipid droplets was markedly increased by addition of norepinephrine but homogenization of the droplets caused loss of norepinephrine responsiveness. These results suggest that the site of the lipolytic action of norepinephrine may be the lipid droplets. When 3T3-L1 fat cell monolayers were exposed to 3H-norepinephrine for 2 or 4 min, many silver grains, representing the localization of labeled norepinephrine, were observed in autoradiograms of cell sections. The grains were found in the cytoplasm and along the margin of the lipid droplets. These results also suggest that norepinephrine may be incorporated into fat cells and exhibit its lipolytic action inside the cells.